The prior art includes devices, such as fire alarms, in which a fusible material melts at an elevated temperature and releases a spring-propelled piston which moves to close an electrical circuit. Such devices are disclosed in U.S. Pat. Nos. 487,519; 821,098; 1,552,105 and 1,685,136. Those structures are not, however, suitable to sever an electrical wire to open the electrical circuit because as the fusible material melts the spring urges the piston to creep toward its closed position. The piston thus slowly moves to its closed position. This is adequate to close an electrical circuit but it does not provide sufficient force to sever a wire to open an electrical circuit.
Devices for severing an electrical wire or the like utilizing a propelled cutting piston are disclosed in U.S. Pat. Nos. 3,003,045; 3,246,396; and 3,640,169. The latter two patents, which disclose means for propelling the cutting piston, utilize an explosive charge to supply sufficient energy to the piston to sever the wire.